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The Segmentum Obscurus, also known as the Segmentum Obscura, is the region of Imperial space to the galactic north of Terra. This Segmentum of the Imperium of Man is the home of the vast Warp Storm and Warp Rift called the Eye of Terror, the primary home for the Forces of Chaos in the Milky Way Galaxy. The Imperial Navy's Segmentum fleet headquarters is located on Cypra Mundi, which also serves as the central headquarters of the Imperial military's Segmentum Command. The Segmentum Obscurus is the area of the Imperium most often under deadly threat by attacks from the Eye of Terror and is also the site for all of Abaddon the Despoiler's Black Crusades. As such, it currently is home to the majority of the Imperium's armed forces, which were most recently mobilised to a state of readiness not seen since the Horus Heresy as a result of the 13th Black Crusade in 999.M41.

The constant turmoil surrounding the Cadian Gate requires the fleets of Battlefleet Obscurus to be far more active than many other Segmentum battlefleets. The whole sector exists in an almost perpetual state of readiness, and as such the demands on manpower and resources are huge. Vast numbers of new, inexperienced ratings frequently have to be drawn up to replace those lost in the constant fighting. However, those that survive the harsh first years of military service around the Cadian Gate rapidly mature to become some of the most capable leaders in the Imperium. As with all highly militarised zones, it is important for the Imperial Navy to maintain substantial reserves of vessels around the Eye of Terror, ready to deploy as reinforcements should the need arise. Whilst it is possible to keep the vessels themselves ready in this manner, Battlefleet Obscura simply does not possess the manpower to keep reserve crews stationed aboard these vessels as well. Instead, whole orbital shipyards are filled with rows of silent, inactive vessels, often representing classes of warship now outdated or scarce.

In times of great need, these ships will be brought into service and crewed with ratings from destroyed or crippled vessels or even with hastily mustered new recruits, meaning the crew is unlikely to have any familiarity with their new vessel. A posting to such a reserve fleet is thus considered an unnerving duty, forcing a rating to enter deep into an unfamiliar vessel which may have lain dormant for centuries. Much superstition surrounds such fleets and perhaps because of this, unusual behaviour among both crew and vessel in these fleets is a rather commonplace occurrence. Reserve fleets are used only reluctantly by the Imperial Navy, and only in the most desperate of circumstances, but an invasion the size of Abaddon's 13th Black Crusade without doubt qualifies as exactly that -- the most dire of circumstances, the most desperate of times.

In a reserve fleet, all vessels are prone to certain unexpected events, such as mass panic or even mutiny amongst the crew, inexplicable noises, sensor readings and sightings or disconcerting setbacks and failure of machinery. Against Chaos fleets, where the followers of the Dark Gods are able to exert their influence to further unsettle their already nervous opponents, unforeseen events can be even more devastating.

The Scarus Sector is one of the closest sectors to the vitally important Fortress World of Cadia and the Cadian Gate, the only predictably stable Warp route that leads out of the Eye of Terror and into the realm of the Imperium. The Scarus Sector was discovered during the middle years of the Great Crusade in the late 30th Millennium. It was named for Captain Scarus of the Word BearersSpace Marine Legion's 52nd Company, whose Strike CruiserShield of Scarus and two other vessels were lost during a savage Warp Storm that occurred during the XVII Legion's sojourn in the Eye of Terror during the quest now known as the Pilgrimage of Lorgar some forty years before the start of the Horus Heresy. The Scarus Sector's capital is the Hive World of Eustis Majorus. The Scarus Sector borders the Calixis Sector to spinward and its known Sub-sectors include Angelus, Antimar, Helican, Kerak and Ophidian. This sector is also known for being the theatre of operation for the renowned InquisitorsGregor Eisenhorn and Gideon Ravenor. The Helican Sub-sector is the site of the Helican Schism, a wave of rebellion, terrorism and sabotage that swept the region in the early 41st Millennium. The Ophidian Sub-sector is noted for being the site of the Ophidian Campaign (also known as the Purge Campaign), in which Battlefleet Scarus wrested control of the Sub-sector from Heretic forces. The campaign lasted from 240.M41 until 338.M41 -- and the Imperial victory represented a considerable achievement on such a short timescale.

In 999.M41, war came without warning to the Scarus Sector, and the opening moves by the Forces of Chaos saw large number of plague-ridden Space Hulks entering Imperial space. Though many were intercepted by the Space Marines and destroyed, some managed to escape the Astartes' attention. Some of these foul vessels of contagion found their way into the Belis Corona Sector, spreading their disease and pestilence amongst the masses throughout the sector. Along with the adjacent Agripinaa Sector, they were some of the first to bear witness to the rise of hordes of infectious, undead zombies that threatened billions of people on many of the Imperial worlds within the sector. This plague was later named the Curse of Unbelief.

While the defence of the Cadian Gate from the Forces of Chaos continued to go badly upon the surface of many worlds in the Cadian System, in particular Saint Josmane's Hope and Solar Mariatus, the war in space was fought more on the Imperium's terms. The Cadian System had been the focus of Abaddon's primary fleet actions, and the Imperium's fortunes there changed from day to day. The situation was highly fluid and the Chaotic naval forces found themselves stretched to exploit the victories they had won.

A fleet the size of which has not been seen since the end of the Gothic War eight standard centuries earlier had been dispatched from Cypra Mundi, and arrived at the world of Belis Corona in preparation for a massive Imperial counter-offensive into the Cadian Gate. The arrival of this vast armada allowed those Imperial vessels that had been fighting continuously since the beginning of the invasion a brief respite. Severely depleted ship’s companies were bolstered through indiscriminate press-ganging, and hasty repairs and refits were undertaken on those vessels most in need of them.

The Imperial reinforcements were split into battlegroups, each tasked with bolstering the defences in a specific sector of the Segmentum Obscurus. The regions around the Eye of Terror encompass many millions of cubic light years, and only by the concentrated application of resources in those areas in most desperate need could the Imperial Navy hope to make inroads and slow, stall, and eventually repel the Chaos warfleets plaguing the region. A small number of Imperial Navy ships won a decisive victory when they managed to hold back a larger Chaos Fleet at the Ilthirium Belt, buying time for Battlefleet Agripinaa to arrive and allow the remaining ships to retreat. This course of action however, left the Agripinaa Sector open to invasion and soon the worlds of this sector and the Belis Corona Sub-Sector was transformed into nightmarish realms of plague and pestilence.

With Warp Storms rendering astropathic communications erratic, the Segmentum Obscuras High Command continued to experience enormous difficulties coordinating command and control of its forces. Communications from the more distant sectors was particularly affected, and Imperial Commanders were advised that those communications they did receive from distant sources were not to be trusted, due to psychodynamic distortion. A particularly implausible report from the Scarus Sector indicated that in addition to Chaos attacks, the defenders there faced the menace of the Orks. After action reports cited Mordax Prime as the initial focus of a massive Ork incursion which occurred at the same time as the Chaos assault was first hitting the Segmentum, and if this were just an isolated case High Command would normally have written off the communication as misinformation caused by inexperienced troops facing an enemy they had no experience with. However, every major star system in the Scarus Sector soon logged similar reports, so they had to be taken seriously until more detailed information was forthcoming.

Abaddon had recruited massed warbands of Orks to act as mercenary auxilia to his Chaos Space Marine Legions for his 13th Black Crusade. Following up on terror raids and cunning feints by the Night Lords and Alpha LegionTraitor Legions many of the Orks banded together into ever-growing tribes that declared themselves as part of the "Green Krusade" (or Kroosade in some sources -- Ork spelling is always a matter of conjecture) in imitation of the grand assault of Chaos. The unstoppable force of the Green Kroosade inundated the Scarus Sector (Skar-Uz to the Orks). The Orks continued putting intolerable pressure on a number of worlds in the Scarus Sector, notably Lethe II and Mordax Prime. The Magi Biologus of the Adeptus Mechanicus believed these foul creatures to be working alongside Abaddon the Despoiler to further their own ends, though they did not consider it to be a true alliance as such. More likely the Orks were taking advantage of the opportunity provided by the Despoiler's invasion to launch new attacks against distracted Imperial targets.

The situation on these worlds was grim indeed. In particular, Mordax Prime could not be allowed to fall into Greenskin hands, for it was a Forge World of prodigious output and its loss would damage the Imperium's war effort. More to the point, the Imperial forces dreaded the monstrosities the Orks might turn the production lines of Mordax Prime to creating. Fifteen Imperial Guard Army Groups and 5 legions of cybernetic Mechanicus Skitarii troops continued to hold Mordax Prime, yet clearly such a large body of men was insufficient to stem the green tide engulfing that Forge World. The Segmentum Obscurus High Command drew up plans to send a further 35 to 40 regiments of the Imperial Guard to the world, drawn from the regional reserve to help bolster Mordax Prime's defences. Yet the Orks had set their sights upon capturing Mordax Prime, broadcasting their designs for the planet they referred to as "MoreDakka Prime" on every Imperial channel. Were it not for the fact that Mordax Prime was the sovereign preserve of the Adeptus Mechanicus, Imperial High Command seriously considered petitioning the Inquisition for an Exterminatus against the world before its vast resources fell to the enemy.

The Orks were able to batter down Imperial defences with sheer doggedness and not a little help from Chaos Space Marines, particularly from the Night Lords and the Black Legion. Now the Orks rule Lethe Eleven and Mordax, along with most of Imbrium and Ulant. Gudrun and Nysa Stromolo stand on the brink of anarchy and the weapon forges of Mordax (rechristened "MoreDakka Prime" by the Orks) are infested with Lootas of the Death Skulls clan, a truly terrifying prospect to the devotees of the Machine God, who know full well the depths of blasphemy against the Omnissiah to which these barbaric xenos will sink.

Battered survivors speak of hundreds of Gargants being built from the debris of war by enslaved servitors and their brutal Greenskin overseers. The squat, menacing forms of more Gargant construction sites climb over the plains of Mordax dairy as Ork Mekaniaks race to be the first to complete their machines. Imperial stategists are now regarding the Green Kroosade as a full-scale Ork WAAAGH! Its Warlords are unknown but it would appear that Ghazghkull and Nazdreg are not among them. Only Thracian Primaris and Elnaur Delta have held firm against the Green Kroosade, bolstered first by the intervention of a Black Templars' Crusade, and later by the stalwart defence mounted by the SalamandersChapter and many other Space Marine contingents. The situation is grim in Scarus. and none can see an end to the infestation given the dire situation across the region.